The present invention relates to an electron gun for a cathode ray tube and a method of assembling the same.
With personalization of computers in recent years, it is desirable to make the display device smaller in thickness and lighter in weight. As a matter of fact, a flat display has already been put to a practical use in a liquid crystal display device, a plasma display, etc. However, these displays are not yet comparable to a cathode ray tube in the size of the display panel, in the fineness and in the manufacturing cost.
For example, an in-line type color cathode ray tube generally comprises an envelope having a panel and a funnel, and an electron gun is arranged within the neck of the funnel. Three electron beams emitted from the electron gun are deflected by a deflecting device mounted on the outside of the funnel and horizontally and vertically scan a phosphor screen formed on the inner surface of the panels through a shadow mask, there by displaying a color picture image on the phosphor screen.
The electron gun includes three cathodes that are linearly arranged, three heaters for heating these cathodes, and a plurality of electrodes, e.g., six electrodes, arranged successively apart from the cathodes. Three electron beams, which are emitted from the cathodes heated by the heaters, are converged on the phosphor screen by an electron lens formed of a plurality of electrodes G1 to G6.
The cathode of a conventional electron gun includes a thin cathode sleeve, a base metal attached to an edge portion of the cathode sleeve on the side of the electrodes, an electron emitting layer formed on the surface of the base metal on the side of the electrodes, a strap attached to the outer circumferential surface of the cathode sleeve, a cylindrical reflector arranged to surround the outer circumferential surface of the cathode sleeve, a cylindrical cathode holder arranged outside the reflector for supporting the cathode sleeve and the reflector via the strap, a support cylinder attached to the outer circumferential surface of the cathode holder, and a support strap attached to the support cylinder.
The heater, which is spirally wound, is inserted into the inside of the cathode sleeve. Both end portions of the heater are attached to a heater tub, and the heater is attached to a heater tub strap via the heater tub. The cathode of the particular construction is supported on a bead glass together with a plurality of electrodes via the heater, the support strap and the heater tub strap.
In the electron gun provided with the cathode-heater portion of the particular construction, the distance between the surface of the electrode G1 on the side of the electrode G2 and the surface of the electron-emitting layer is set at 0.5 mm, the distance between the surface of the electron-emitting layer and the lower end of the cathode holder is set at 9.5 mm, and the distance between the lower end of the cathode holder and the lower end of the heater tub is set at 6.0 mm. Therefore, the entire length of the cathode-heater portion is 16 mm, which is about 30% of the electron gun having an entire length of 50 mm. In other words, the length of the cathode-heater portion occupies a considerably large proportion of the entire length of the electron gun.
In general, the operating temperature of a cathode using an oxide of an alkaline earth metal as an electron-emitting material, i.e., an oxide cathode, is about 830.degree. C., and the heater power for heating the cathode to the operating temperature is 0.7 W. It follows that a heater power of 2.1 W is required in a color cathode ray tube equipped with three cathodes.
It should also be noted that, in an electron gun equipped with the cathode-heater portion of the construction described above, it takes about 10 seconds for the displayed image to be stabilized after supply of the heater power.
To reiterate, in the electron gun included in the conventional cathode ray tube, the cathode-heater portion occupies about 30% of the entire length of the electron gun. Naturally, it is necessary to shorten the cathode-heater portion of the electron gun in order to decrease the entire length of the cathode ray tube in the axial direction of the tube. Needless to say, it is important to decrease the entire length of the cathode ray tube in order to decrease the thickness of the display device.
It should also be noted that a plurality of color TV receivers and personal computers are installed in a single family in recent years, leading to a large power consumption in the family. This makes it important to decrease the heater power of the electron gun.
It is also important to achieve a rapid start-up of a cathode ray tube. In recent years, the color cathode ray tube employs a pre-heating system, with the result that, if a main power source is kept turned on, a predetermined current is kept allowed to flow through the heater. It follows that a stable picture image can be obtained promptly. However, the pre-heating system is not desirable in terms of the power saving. Also, the pre-heating system certainly permits obtaining a stable picture image promptly. However, about 10 seconds are required for obtaining a stable picture image, which is not quite satisfactory.
A cathode-heater structure adapted for overcoming the above-noted problems is disclosed in, for example, U.S. Pat. No. 5,105,908. It is taught that a heater of a predetermined pattern consisting of an anisotropically heat decomposable graphite film is formed on an insulating substrate made of boron nitride that is anisotropically heat decomposable. The heater is thin, i.e., about 1 mm thick, making it possible to decrease the entire length of the cathode-heater structure and to further improve the start-up speed. However, the cathode-heater structure is constructed to be adapted for use in a large and high power electron tube such as a klystron or a traveling-wave tube, but is not constructed to be adapted for use in a small and low power apparatus that can be obtained by mass production such as a cathode ray tube.